"Tipping" on ebay

It's Karma.

I haven't done it for Ebay but I do on craigslist if I see someone selling something wayyyy under what it should be. There's too many annoying cheapskates out there who try to take advantage of your patience and money as a seller so when I see someone being realistic I'll give them what they ask and thank them. I even give stuff away regularly to keep the "balance".